def permutations(iterable, r=None):
      # permutations('ABCD', 2) --> AB AC AD BA BC BD CA CB CD DA DB DC
      # permutations(range(3)) --> 012 021 102 120 201 210
      pool = tuple(iterable)
      n = len(pool)
      r = n if r is None else r
      if r > n:
          return
      indices = range(n)
     cycles = range(n, n-r, -1)
     yield tuple(pool[i] for i in indices[:r])
     while n:
         for i in reversed(range(r)):
             cycles[i] -= 1
             if cycles[i] == 0:
                 indices[i:] = indices[i+1:] + indices[i:i+1]
                 cycles[i] = n - i
             else:
                 j = cycles[i]
                 indices[i], indices[-j] = indices[-j], indices[i]
                 yield tuple(pool[i] for i in indices[:r])
                 break
         else:
             return